A visible colorimetric pH sensitive chemosensor based on azo dye of benzophenone

Article abstract of DOI:10.1016/j.dyepig.2011.03.003

A visible colorimetric pH sensitive chemosensor (Dye A) based on a benzophenone azo dye was designed and synthesized. This chemosensor had a large wavelength shift (>170 nm) and showed excellent sensitivity in the range of pH from 7.9 to 9.3. In addition,

Full text of DOI:10.1016/j.dyepig.2011.03.003

Dyes and Pigments 91 (2011) 294e297
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
A visible colorimetric pH sensitive chemosensor based
on azo dye of benzophenone
*
Yunming Wang, Bingtao Tang , Shufen Zhang
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Zhongshan Road, Dalian 116012, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A visible colorimetric pH sensitive chemosensor (Dye A) based on a benzophenone azo dye was designed
and synthesized. This chemosensor had a large wavelength shift (>170 nm) and showed excellent
sensitivity in the range of pH from 7.9 to 9.3. In addition, the possible mechanism of the chemosensor
with remarkable change of visible color was established by the 1H-NMR- and 13C-NMR-titration
experiments. This pH chemosensor has a fast response time and is fully reversible, which is important for
pH chemosensors in practical applications. In this study, Dye A was successfully applied to determine the
pH of simulated small intestine fluid.
Received 3 December 2010
Received in revised form
28 February 2011
Accepted 2 March 2011
Available online 16 March 2011
Keywords:
Ó 2011 Elsevier Ltd. All rights reserved.
Highly sensitive
pH chemosensor
Large wavelength shift
Colorimetric sensor
Visible color change
Naked eye
1. Introduction
2. Experimental
The pH plays significant roles in agriculture [1], industry [2,3],
human health [4,5], and, more particularly, bio-gland secretion
of fluid [6]. So, the methods for visualizing Hþ ions would
be powerful tools to examine the concentration of Hþ ion-
signaling mechanisms in detail and might eventually be useful
for diagnosis as well. Although over the past decade, fluorescence
detection methods for pH have been well reported [7e11], visible
ratiometric detection methods are widely welcome for pH sensing
because the process of detecting the analyte can be observed with
the naked eyes [12,13]. However, most colorimetric chemosensors
have a weaker pH sensitivity than fluorescent probes [14e16]
and can cause measurement errors due to poor color change.
Therefore, a highly pH sensitive colorimetric chemosensor having
a significant color change would be very useful in the field of
physiological fluids.
2.1. Materials
All chemicals were analytical grade and purchased from Sino-
pharm Chemical Reagent Co., Ltd., China. The purification of the
dye was performed by conventional column chromatography on
silica-gel absorbent (200e300 Å, Qingdao Makall Group Company,
China). Ethyl acetate-petroleum ether (60e90) mixtures were used
as the elution. Deionized water was redistilled before use. Dimethyl
sulphoxide (DMSO) was HPLC grade.
2.2. Preparation and characterization
The preparation of Dye A based on azo dye of benzophenone
was carried out by diazo-coupling reaction as shown in Scheme 1.
This communication describes the design and synthesis of
a visible colorimetric sensitive chemosensor (Dye A) derived from
benzophenone (Fig. 1).
2.2.1. Procedure for the preparation of Dye A
A mixture of 4-nitroaniline (1.38 g, 10 mmol), 3.6 ml 37% or
concentrated hydrochloric acid and 25 ml H2O was stirred vigorously
for 0.5 h at 70 ꢀC. The resultant solution was cooled below 5 ꢀC with
the aid of an ice bath and stirred as NaNO2 (0.73 g, 10.5 mmol) in 5 ml
H2O was added rapidly. The reaction mixture was stirred at 5 ꢀC until
a clear solution was formed. The solution was added to 50 ml H2O
containing compound 2-hydroxy-4-methoxybenzophenone (2.28 g,
10 mmol) and Na2CO3 (2.40 g, 45.4 mmol) while keeping the
* Corresponding author. Tel.: þ86 13154116360.
E-mail address: tangbt@dlut.edu.cn (B. Tang).
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.03.003

Y. Wang et al. / Dyes and Pigments 91 (2011) 294e297
295
Fig. 1. UV/Vis absorption spectra of dye (5.33 ꢂ 10ꢁ5 mol Lꢁ1) in DMSO solution containing 10% water.
temperature not higher than 5 ꢀC during the addition, and the
resulting solution was stirred for 2.5 h at 5 ꢀC. Then, the pH of reaction
solution was adjusted to 7.0 by adding 10% Na2CO3 aqueous solution.
The solid was filtered, washed with 50 ml H2O, and dried to give
a yellow solid. The Dye A was obtained by column chromatograph
silica gel using ethyl acetate-petroleum ether (60e90) mixtures as the
eluent, yield: 77.8%. 1H-NMR (400 MHz, DMSO-d6, ppm):
(s, 3H, CH3), 6.86 (s,1H, CH), 7.54e7.58 (t, 2H, CH), 7.65e7.69 (t, H, CH),
d
¼ 4.06
Scheme 1. Synthesis of Dye A from 2-hydroxy-4-methoxybenzophenone.
Fig. 2. (a) Nonlinear fitting of the pH-dependent value of absorbance
e 402 nm; (b) Nonlinear fitting of the pH-dependent value of absorbance e 571 nm; (c) The relationship of
absorbance intensity ratio (Vis571 nm/Vis402 nm) and pH; (d) UV/Vis absorption spectra of dye (5.33 ꢂ 10ꢁ5 mol Lꢁ1) with different pH in DMSO solution containing 10% water; Inset:
the color change of Dye A (5.33 ꢂ 10ꢁ5 mol Lꢁ1) with different pH in DMSO-H2O 9:1 (V/V).
7.72e7.74 (d, 2H, CH), 7.85 (s, H, CH), 7.93e7.95 (d, 2H, CH), 8.38e8.40
13
(d, 2H, CH), 11.86 (s, H, OH); C-NMR (400 MHz, DMSO-d6, 25 ꢀC,
TMS):
129.08, 128.50, 125.00, 123.17, 119.63, 116.40, 100.87, 56.56; Q-TOFMS:
ꢁ1 calculated: 377.0933, measured: 377.0941.
d
¼ 196.94, 164.84, 162.43, 155.57,147.80, 137.50, 134.64, 132.70,
M
2.3. Methods
All absorption measurements of solution were performed in
a 1 ꢂ1 ꢂ4 cm quartz cuvette (sight optical path length, 1 cm)
(Starna), using a HP-8453 spectrophotometer. The pH measurements
Fig. 3. The pH 1H-NMR titration of dye (3.0 ꢂ 10ꢁ6 mol Lꢁ1) in DMSO: (a) dye only;
(b) 1 þ1.00 equiv of TEA.

296
Y. Wang et al. / Dyes and Pigments 91 (2011) 294e297
Scheme 2. Representative mechanism of the chemosensor based on the phenolic hydroxyl-group providing electron and the nitro-group absorbing electron.
the ratio was 0.42 by changing the pH value of 0.1 in the range of pH
from 8.5 to 10.0. Therefore, the new Dye A had a higher sensitivity in
the narrow pH range in terms of pH detection. The pKa value of Dye
A was 9.20 calculated according to a literature method [15], which
was suitable for alkaline fluid in vivo.
As shown in Fig. 2, there were some important features in the
spectra of this new dye: a remarkably visible color change and
no mirror image relationship between peaks. These features may
result from an intramolecular charge transfer or rearrangement
versus isomerization. To elucidate the mechanism of the remark-
ably visible color change, 1H-NMR-titration experiments were
conducted. As shown in Fig. 3, a single peak corresponding to the
Fig. 4. The pH 13C-NMR titration of dye (3.0 ꢂ 10ꢁ6 mol Lꢁ1) in DMSO: (a) dye only; (b)
1 þ1.00 equiv of TEA.
were made with a pHS-25 digital pH-meter (Shanghai Lei Ci Device
Works, Shanghai, China) with a combined E-201-C electrode. All
experiments were performed at 293 K. The experimental curves
were analyzed to extract the pKa values according to a literature
method [15].
hydroxybenzene proton (
the increase of TEA in the range of 0.0e1.0 equiv. Concomitantly,
the hydrogen in the -position gradually displayed an apparent
a position) at 11.84 ppm disappeared with
b
highfield shift from 6.86 ppm to 6.13 ppm. This indicated that
the opened-ring form (Scheme 2) of Dye A became Dye B or Dye C
in the examined solution. To further elucidate the structural
mechanism of mutual transformation of Dye A (Scheme 2), the
13C-NMR-titration (Fig. 4) and HMBC (Fig. 5a and b) was conducted.
As shown in Fig. 4, the peak corresponding to No. 7 at 164.83 ppm
3. Results and discussion
The UV/Vis titrations for pH with triethylamine (TEA) were
performed in a DMSO solution containing 10% water with a che-
mosensor concentration of 5.33 ꢂ 10ꢁ5 mol Lꢁ1. Dye A showed an
obvious color change visible to naked eyes from yellow (pH ¼ 7.9)
to Lyons blue (pH ¼ 9.3) (Figs. 1 and 2d). As the pH increased, the
absorbance intensity decreased at 401 nm (Fig. 2a), and increased
at 571 nm (Fig. 2b). The isobestic point observed in both cases
indicated equilibrium between two species as pH increased. Fig. 2c
shows clearly that a significant change appeared for the ratiometric
absorption measurements with the increase of pH, and the ratio of
the absorbance intensity at 571 nm vs 402 nm was nearly a constant
(R2 ¼ 0.98152). Compared with common colorimetric dyes [17,18],
disappeared, however, from HMBC
a new peak appeared at
175.40 ppm with the increase of the TEA (1.0 equiv), which was No. 7
by the analysis from HMBC. Therefore, the peak of No. 7 displayed an
apparent downfield shift (from peak centered at 164.83e175.40 ppm,
the Dd ¼ 10.57 ppm, upon addition of 1.0 equiv of TEA) which further
proved that the coordination was changed from hydroxybenzene to
benzoquinone. Concomitantly, these carbons of No. 6, 10, 12 and 15
also displayed an apparent shift (Dd S 2.32 ppm). The peak of No.15
was shown in Fig. 5 with increasing the volume of TEA to 1.0 equiv.
One important structural change accompanying an oxygen anion
Fig. 5. (a) 13Ce1H HMBC of Dye A (Dye A: NEt3 ¼ 1:1); (b) 13Ce1H HMBC of Dye A.

Y. Wang et al. / Dyes and Pigments 91 (2011) 294e297
297
Fig. 6. Reversibility of protonation and color change.
Fund of Ministry of Education of China (200801411032);
The National Natural Science Foundation of China (20804007);
The Program for Changjiang Scholars and Innovative Research
Team in University (IRT0711).
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showed that the arrangement of the azo bond was transferred from
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In a test of reversibility, the pH value was changed from 8.1 to
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H2O (9:1, v/v). An obvious color change from yellow to purple was
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Acknowledgments
This work was financially supported by the State Key Program of
National Natural Science Foundation of China (20836001); Doctoral